fmemopen

SYNOPSIS
#include <stdio.h>
FILE *fmemopen(void *buf, size_t size, const char *mode);
FILE *open_memstream(char **ptr, size_t *sizeloc);
#include <wchar.h>
FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fmemopen(), open_memstream(), open_wmemstream():
Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
DESCRIPTION
The fmemopen() function opens a stream that permits the access speci-
fied by mode. The stream allows I/O to be performed on the string or
memory buffer pointed to by buf. This buffer must be at least size
bytes long.
The argument mode is the same as for fopen(3). If mode specifies an
append mode, then the initial file position is set to the location of
the first null byte ('\0') in the buffer; otherwise the initial file
position is set to the start of the buffer. Since glibc 2.9, the let-
ter 'b' may be specified as the second character in mode. This pro-
vides "binary" mode: writes don't implicitly add a terminating null
byte, and fseek(3) SEEK_END is relative to the end of the buffer (i.e.,
the value specified by the size argument), rather than the current
string length.
When a stream that has been opened for writing is flushed (fflush(3))
or closed (fclose(3)), a null byte is written at the end of the buffer
if there is space. The caller should ensure that an extra byte is
available in the buffer (and that size counts that byte) to allow for
this.
Attempts to write more than size bytes to the buffer result in an
error. (By default, such errors will be visible only when the stdio
buffer is flushed. Disabling buffering with setbuf(fp, NULL) may be
useful to detect errors at the time of an output operation. Alterna-
tively, the caller can explicitly set buf as the stdio stream buffer,
at the same time informing stdio of the buffer's size, using set-
buffer(fp, buf, size).)
In a stream opened for reading, null bytes ('\0') in the buffer do not
cause read operations to return an end-of-file indication. A read from
the buffer will only indicate end-of-file when the file pointer
advances size bytes past the start of the buffer.
When the stream is closed (fclose(3)) or flushed (fflush(3)), the loca-
tions pointed to by ptr and sizeloc are updated to contain, respec-
tively, a pointer to the buffer and the current size of the buffer.
These values remain valid only as long as the caller performs no fur-
ther output on the stream. If further output is performed, then the
stream must again be flushed before trying to access these variables.
A null byte is maintained at the end of the buffer. This byte is not
included in the size value stored at sizeloc.
The stream's file position can be changed with fseek(3) or fseeko(3).
Moving the file position past the end of the data already written fills
the intervening space with zeros.
The open_wmemstream() is similar to open_memstream(), but operates on
wide characters instead of bytes.
RETURN VALUE
Upon successful completion fmemopen(), open_memstream() and open_wmem-
stream() return a FILE pointer. Otherwise, NULL is returned and errno
is set to indicate the error.
VERSIONS
fmemopen() and open_memstream() were already available in glibc 1.0.x.
open_wmemstream() is available since glibc 2.4.
CONFORMING TO
POSIX.1-2008. These functions are not specified in POSIX.1-2001, and
are not widely available on other systems.
POSIX.1-2008 specifies that 'b' in mode shall be ignored. However,
Technical Corrigendum 1 adjusts the standard to allow implementation-
specific treatment for this case, thus permitting the glibc treatment
of 'b'.
NOTES
There is no file descriptor associated with the file stream returned by
these functions (i.e., fileno(3) will return an error if called on the
returned stream).
BUGS
In glibc before version 2.7, seeking past the end of a stream created
by open_memstream() does not enlarge the buffer; instead the fseek(3)
call fails, returning -1.
If size is specified as zero, fmemopen() fails with the error EINVAL.
It would be more consistent if this case successfully created a stream
that then returned end of file on the first attempt at reading. Fur-
thermore, POSIX.1-2008 does not specify a failure for this case.
Specifying append mode ("a" or "a+") for fmemopen() sets the initial
file position to the first null byte, but (if the file offset is reset
to a location other than the end of the stream) does not force subse-
The glibc 2.9 addition of "binary" mode for fmemopen() silently changed
the ABI: previously, fmemopen() ignored 'b' in mode.
EXAMPLE
The program below uses fmemopen() to open an input buffer, and
open_memstream() to open a dynamically sized output buffer. The pro-
gram scans its input string (taken from the program's first command-
line argument) reading integers, and writes the squares of these inte-
gers to the output buffer. An example of the output produced by this
program is the following:
$ ./a.out '1 23 43'
size=11; ptr=1 529 1849
Program source
#define _GNU_SOURCE
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
FILE *out, *in;
int v, s;
size_t size;
char *ptr;
if (argc != 2) {
fprintf(stderr, "Usage: %s <file>\n", argv[0]);
exit(EXIT_FAILURE);
}
in = fmemopen(argv[1], strlen(argv[1]), "r");
if (in == NULL)
handle_error("fmemopen");
out = open_memstream(&ptr, &size);
if (out == NULL)
handle_error("open_memstream");
for (;;) {
s = fscanf(in, "%d", &v);
if (s <= 0)
break;
s = fprintf(out, "%d ", v * v);
if (s == -1)
handle_error("fprintf");
This page is part of release 3.54 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
GNU 2012-04-28 FMEMOPEN(3)